It has been a goal in the field of lubricating oils to improve the properties of lubricating oils, including the viscosity-temperature characteristic and heat and oxidation stability, by addition of various additives such as highly refined mineral oils to the lubricating base oils (see Patent documents 1-3).
For example, lubricating oils used for internal combustion engines such as automobile engines must exhibit heat and oxidation stability to withstand used for long periods under severe conditions. In order to ensure heat and oxidation stability for conventional internal combustion engine lubricating oils, it is common to use high performance base oils which include highly refined base oils as represented by hydrocracked mineral oils and synthetic oils, and to mix with the base oils peroxide-decomposing sulfur-containing compounds such as zinc dithiophosphate (ZDTP) or molybdenum dithiocarbaminate (MoDTC), or ashless antioxidants such as phenolic or amine antioxidants (for example, see Patent documents 1 and 4-6).
In recent years it has become a major goal to achieve energy reduction, i.e. improved fuel efficiency, for automobiles, construction equipment, agricultural machinery and the like in the light of environmental issues such as reducing carbon dioxide gas emissions, and it is strongly desirable to devise means of further reducing energy used by power train devices such as transmissions and final reduction gears. One means for achieving increased fuel efficiency in power train devices is to lower the viscosity of the lubricating oil to reduce stirring resistance and friction resistance on the sliding surfaces. Typical transmitting devices such as automobile automatic transmissions and continuously variable transmissions comprise torque converters, wet clutches, gear bearing mechanisms, oil pumps, overpressure control mechanisms and the like, while manual transmissions and final reduction gears comprise gear bearing mechanisms, and it is possible to realize fuel savings by lowering the viscosity of the lubricating oils used therein to lower the stirring resistance and friction resistance, thus improving power transmission efficiency. However, lowering the viscosity of lubricating oils also leads to lower lubricity (antiwear property, anti-seizing properties and fatigue life), which can cause problems in transmission devices and the like. When phosphorus-containing extreme-pressure agents are added to ensure antiwear property for low-viscosity lubricating oils, the fatigue life is significantly shortened. Sulfur-containing extreme-pressure agents are effective for improving fatigue life, but as is generally known, the effect of the viscosity of the lubricating base oil is greater than that of the additives in low-viscosity lubricating base oils. In order to ensure lubricity with low-viscosity lubricating oils for the purpose of achieving fuel savings, it has therefore been attempted to optimize the combination of phosphorus-containing extreme-pressure agents and sulfur-containing extreme-pressure agents added to lubricating base oils (for example, see Patent documents 7 and 8).
[Patent document 1] Japanese Unexamined Patent Publication HEI No. 4-36391
[Patent document 2] Japanese Unexamined Patent Publication HEI No. 4-68082
[Patent document 3] Japanese Unexamined Patent Publication HEI No. 4-120193
[Patent document 4] Japanese Unexamined Patent Publication SHO No. 63-223094
[Patent document 5] Japanese Unexamined Patent Publication HEI No. 8-302378
[Patent document 6] Japanese Unexamined Patent Publication HEI No. 9-003463
[Patent document 7] Japanese Unexamined Patent Publication No. 2004-262979
[Patent document 8] Japanese Unexamined Patent Publication No. 2004-262980